High-frequency ultrasonic sensors are an important sensing technology in structural health monitoring applications. Compared with the traditional PZT transducer as ultrasonic sensors, novel ultrasonic sensors based on optical methods such as micro-ring resonators have gained increased attention. These micro-rings can be as small as a few microns in diameter, which improves their sensitivity to high-frequency ultrasound. In principle, acoustic waves irradiating the micro-ring induce strain, changing the dimensions and refractive index of the waveguides via the elasto-optic effect. This leads to a change of the guided whispering gallery modes (WGMs), which are extremely sensitive to change in the ring radius induced by the ultrasound strain field. Based on our prior research, here we present an integrated high-frequency ultrasonic sensor array based on optical micro-ring resonator array fabricated by direct laser writing. The fabrication has been optimized to provide high optical quality factor to ensure high detection sensitivity. The experiments demonstrate the potential of the polymer micro-ring resonator working as a high-performance ultrasonic sensor. Applications of the integrated ultrasonic sensor array for acoustic-emission ultrasound detection are shown.
Damage in civil, aerospace, and mechanical structures caused by crack growth and impact loading generate transient ultrasonic waves whose frequency and amplitude can reveal the underlying structural health condition. Hence, it is necessary to find a useful tool based on ultrasonic detection for structural health monitoring. Recently, smart sensors based on gratings such as fiber Bragg gratings (FBGs) have been shown to be suitable to detect such acoustic waves in structural health monitoring applications. However, the fiber-based gratings as the ultrasonic sensor has limited sensitivity to high frequency ultrasound detection due to a specific grating length and a finite spectrum width. To eliminate this limitation, one improvement has been made by using phase shift FBGs due to their special filtering characteristics. The phase shift FBGs can have a narrower spectral width, which will significantly improve the detection sensitivity. Another big improvement, for example Bragg grating waveguide (BGW) sensor, is to optimize the grating structure using different materials. In this work, we describe a 3D printed-polymer BGW sensor for ultrasound detection fabricated through a two-photon polymerization process. The design and fabrication have been optimized for high detection sensitivity. The results demonstrate the potential application of BGW devices for high-sensitivity ultrasound detection.
In this paper, we demonstrate the fabrication of a chemical sensor for 2,4-dinitrotoluene (DNT), based on an opticalfiber- microsphere coated with upconversion nanocrystals functionalized with layers of polyelectrolytes - poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). The design consists of a microsphere, which supports whispering-gallery-modes (WGM), coupled to an optical fiber. The NaYF<sub>4</sub>-Yb<sup>3+</sup>,Er<sup>3+</sup> nanocrystals have a bright fluorescence around 550 nm and 650 nm when irradiated with 980 nm, which is enhanced by the WGM. When functionalized with PAA/PAH layers, these nanocrystals can be coated on the microsphere with control over layer thickness. The presence of DNT on the surface of the microsphere quenches the fluorescence as the absorption spectrum of DNT has peaks in 500 - 600 nm. The effect of concentration of the analyte on the magnitude of quenching has been studied. The paper discusses the design, fabrication and characterization of the chemical sensor.
With the development of photoacoustic technology in recent years, ultrasound-related sensors play a vital role in a number of areas ranging from scientific research to nondestructive testing. Compared with the traditional PZT transducer as ultrasonic sensors, novel ultrasonic sensors based on optical methods such as micro-ring resonators have gained increasing attention. The total internal reflection of the light along the cavity results in light propagating in microcavities as whispering gallery modes (WGMs), which are extremely sensitive to change in the radius and refractive index of the cavity induced by ultrasound strain field. In this work, we present a polymer optical micro-ring resonator based ultrasonic sensor fabricated by direct laser writing optical lithography. The design consists of a single micro-ring and a straight tapered waveguide that can be directly coupled by single mode fibers (SMFs). The design and fabrication of the printed polymer resonator have been optimized to provide broad bandwidth and high optical quality factor to ensure high detection sensitivity. The experiments demonstrate the potential of the polymer micro-ring resonator to works as a high-performance ultrasonic sensor.
Damages such as cracking or impact loading in civil, aerospace, and mechanical structures generate transient ultrasonic waves, which can be used to reveal the structural health condition. Hence, it is necessary to find a practical tool based on ultrasonic detection for structural health monitoring. In this work, we describe an intelligent fiber-optic ultrasonic sensing system, which is designed based on a fiber Bragg grating (FBG) and a reflective semiconductor optical amplifier (RSOA) used as an adaptive source, and demodulated by an adaptive photorefractive two wave mixing (TWM) technique without any active compensation of quasi-static strains and temperature. As the wavelength of the FBG shifts due to the excited ultrasonic waves, the wavelength of the optical output from the fiber cavity laser shifts accordingly. With regard to the shift of the FBG reflective spectrum, the adaptivity of the RSOA-based laser is analyzed theoretically and verified by the TWM demodulator. Additionally, due to the response time of the photorefractive crystal, the TWM demodulator is insensitive to low frequency-FBG spectral shift. The results demonstrate that this proposed FBG ultrasonic sensing system has high sensitivity and can respond the ultrasonic waves into the megahertz frequency range, which shows a potential for acoustic emission detection in practical applications.
Use of long period gratings (LPGs) formed in grapefruit photonic crystal fiber (PCF) with thin-film overlay coated on the inner surface of air holes for gas sensing is demonstrated. The finite-element method was used to numerically simulate the grapefruit PCF–LPG modal coupling characteristics and resonance spectral response with respect to the refractive index of thin-film inside the holey region. A gas analyte-induced index variation of the thin-film immobilized on the inner surface of the holey region of the fiber can be observed by a shift of the resonance wavelength. As an example, we demonstrate a 2,4-dinitrotoluene (DNT) sensor using grapefruit PCF–LPGs. The sensor exhibits a wavelength blue-shift of ∼820 pm as a result of exposure to DNT vapor with a vapor pressure of 411 ppbv at 25°C, and a sensitivity of 2 pm ppbv−1 can be achieved.
The detection of explosives and their residues is of great importance in public health, antiterrorism and homeland
security applications. The vapor pressures of most explosive compounds are extremely low and attenuation of the
available vapor is often great due to diffusion in the environment, making direct vapor detection difficult. In this paper, a
photonic-microfluidic integrated sensor for highly sensitive 2,4,6-trinitrotoluene (TNT) detection is described based on
an in-fiber Mach-Zehnder interferometer (MZI) in a photonic crystal fiber (PCF). A segment of PCF is inserted between
standard single-mode fibers (SMF) via butt coupling to form a modal interferometer, in which the cladding modes are
excited and interfere with the fundamental core mode. Due to butt coupling, the small air gap between SMF and PCF
forms a coupling region and also serves as an inlet/outlet for the gas. The sensor is fabricated by immobilizing a chemo-recognition
coating on the inner surface of the holey region of the PCF, which selectively and reversibly binds TNT
molecules on the sensitized surface. The sensing mechanism is based on the determination of the TNT-induced
wavelength shift of interference peaks due to the refractive index change of the holey-layer. The sensor device therefore
is capable of field operation.
Smart sensors based on Optical fiber Bragg gratings (FBGs) are suitable for structural health monitoring of dynamic strains in civil, aerospace, and mechanical structures. In these structures, dynamic strains with high frequencies reveal acoustic emissions cracking or impact loading. It is necessary to find a practical tool for monitoring such structural damages. In this work, we explore an intelligent system based on a reflective semiconductor optical amplifier (RSOA)- FBG composed as a fiber cavity for measuring dynamic strain in intelligent structures. The ASE light emitted from a RSOA laser and reflected by a FBG is amplified in the fiber cavity and coupled out by a 90:10 coupler, which is demodulated by a low frequency compensated Michelson interferometer using a proportional-integral-derivative (PID) controller and is monitored via a photodetector. As the wavelength of the FBG shifts due to dynamic strain, the wavelength of the optical output from the laser cavity shifts accordingly, which is demodulated by the Michelson Interferometer. Because the RSOA has a quick transition time, the RSOA- FBG fiber cavity shows an ability of high frequency response to the FBG reflective spectrum shift, with frequency response extending to megahertz.
Corrosion of steel is one of the most important durability issues in reinforced concrete (RC) structures because aggressive ions such as chloride ions permeate concrete and corrode steel, consequently accelerating the destruction of structures, especially in marine environments. There are many practical methods for corrosion monitoring in RC structures, mostly focusing on electrochemical-based sensors for monitoring the chloride ion which is thought as one of the most important factors resulting in steel corrosion. In this work, we report a fiber-optic chloride chemical sensor based on long period gratings inscribed in a photonic crystal fiber (PCF) with a chloride sensitive thin film. Numerical simulation is performed to determine the characteristics and resonance spectral response versus the refractive indices of the analyte solution flowing through into the holes in the PCF. The effective refractive index of the cladding mode of the LPGs changes with variations of the analyte solution concentration, resulting in a shift of the resonance wavelength, hence providing the sensor signal. This fiber-optic chemical sensor has a fast response, is easy to prepare and is not susceptible to electromagnetic environment, and can therefore be of use for structural health monitoring of RC structures subjected to such aggressive environments.